Southern Plains Heavy rain and Flooding

Transcription

1 Abstract: Southern Plains Heavy rain and Flooding By Richard H. Grumm National Weather Service State College, PA A severe weather event affected the southern Plains on 31 May The severe weather event transitioned into a heavy rainfall and flash flood event as the collective convective elements produced cold pools and a mesohigh. This slow moving mesohigh with deep moisture and strong low-level southerly flow produced an MCS with heavy rains and flash flooding over eastern Oklahoma. Observed precipitation data showed that the pattern also produced heavy rains over Missouri and Arkansas during this period of time. The overall pattern, with a deep trough to the west and a strong ridge to the east is a well-known pattern for heavy rainfall, and at time for severe weather. The models forecast the pattern relatively well to include a dry-line with low precipitable water over the western Plains. The surge of high precipitable water air ahead of this feature like allowed models and ensembles to predict the possibility of convective rainfall with rainfall amounts over 50 to 100 mm, well below the local maximum rainfall values observed, but reasonable over the broader region of interest. This paper documents the pattern and the forecasts by the SREF of the southern Plains heavy rainfall of 31 May to 1 June 2013.

2 1. Overview The convective event of 31 May 2013 will likely be remembered by many people for the deadly El Reno EF5 tornado, not the heavy rainfall that affected central Oklahoma as the supercell event transitioned into a heavy rainfall event (Fig. 1). At least 6 professional storm chasers were caught in the storm, 3 of whom did not survive. Reports suggest that 4 of the 9 fatalities related the El Reno tornado were storm chasers. There will likely be considerable research into the El Reno supercell and the attendant EF5 tornado. But this paper will focus on the heavy rains that affected central Oklahoma, Missouri and Arkansas on 31 May to 1 June The heavy rain and accompanying severe weather were associated with a deep trough which moved out the Rocky Mountains (Fig. 2) and a sharp 500 hpa ridge over eastern North America. The deep southerly flow between these two systems brought a surge of relatively high precipitable water air into the southern and central Plains (Fig. 3). The 500 hpa short-wave coming off the Rockies brought the dry-line layer (Fawbush and Miller 1954; Carlson and Ludlum 1968; Banacos and Ekster 2010) farther east into the Plains which likely contributed to the evolution of the convection and severe convection observed from Oklahoma to Illinois (Fig. 04). Studies on heavy rainfall and extreme heavy rainfall (Schumacher and Johnson 2005) indicated that most extreme heavy rainfall events over the United States east of the Rocky Mountains are convectively driven and over 65% of these events are associated with Mesoscale Convective System (MCS). Most of the remaining events (26.7%: See Table 1 Schumacher and Johnson 2005) were associated with synoptic systems, which had embedded convection within them (See Table 2. Schumacher and Johnson 2005). Examining MCS related heavy rainfall they found that training was the dominate (31.6%) form of heavy rainfall producing MCS activity. This paper will examine the heavy rainfall over the Plains on 31 May through 1 June The focus is on the pattern and anomalies along with forecasts of the evolution of predicted heavy rainfall in the NCEP guidance. The convective nature of this event would imply poor predictability as most of the rain fell as a result of deep convection. 2. Data and Methods The larger scale pattern was reconstructed using the Climate Forecast System (Saha et. 2010). The means and standard deviations were used based on the method described by Hart and Grumm (2001) suing the NCEP/NCAR reanalysis data (Kalnay et al. 1996). Daily values of raw CFS data were compared to the 21-day centered means and standard deviations. 3. Pattern and precipitation

3 The large scale 500 hpa pattern (Fig. 2) showed the strong trough moving out of the Rocky Mountains into the Plains with a ridge to the east. The southerly flow brought a plume of deep moisture into the region (Fig. 3) and a band of strong low-level southerly flow (Fig. 5). The strong trough to the west and ridge to the east is a known pattern conducive for heavy rainfall (Bodner et al. 2011). The pattern, with the dry-line (Fig. 3) and elevated mixed layer is also a pattern conducive for severe weather. But the open Gulf of Mexico between the trough and ridge is a well-known pattern for heavy rainfall as described by Junker et al (1999) and Bodner et al. (2011). The value of the pattern and anomalies of key fields has been shown to be of value in predicting heavy rainfall events (Junker et al. 2009). The large scale pattern on 31 May into 1 June 2013 support the potential for heavy rainfall over portions of the southern and central Plains. The evolution of the rainfall in 12-hour increments (Fig. 6) shows the Missouri had several successive period of heavy rainfall during each of the 12-hour periods, though the regions of heavy rain were not constant. The heavy rains in Arkansas were primarily focused with convection from 1200 UTC 30 May through 1200 UTC 31 May 2013 (Fig. 6a-b) with the heaviest rainfall clearly falling in the UTC period of 31 May In Oklahoma, the heavy rainfall near Oklahoma City was focused in the 12-hour period from UTC 01 June 2013 (Fig. 6d). These rainfall data combined with the pattern show that the heavy rain fell in a pattern well known to produce or be associated with heavy rain. The heaviest rainfall based on rainfall data and the NMQ Q2 site during this event was observed over Oklahoma were some locally heavy rainfall amounts exceeded 10 inches. Most of the rain in Oklahoma (Fig. 6) fell in a 3-6 hour period after the severe weather during the afternoon and evening hours of 31 May An MCS developed with training echoes and in all likelihood a meso-high developed at the surface 1. The KTLX radar showing the 0.5 degree elevation angle of base velocity and reflectivity is shown in Figure 7. At 0109 UTC (Fig. 7a) there were some severe storms still visible on radar and the radar showed an organized outflow boundary from east of KTLX back to the south west. Most of the enhanced convection is located over well over the cool air. Northwest of the radar the winds were mainly inbound at low-levels and outbound farther to the north and west. The outflow boundary at 0109 UTC (Fig. 7a) shows up well in the velocity data with no real signal in the reflectivity data. This boundary when viewed with looping developed as a result of earlier severe convection. New cells developed along a line and moved over in a generally eastward direction. The data at 0118 UTC showed the outflow farther south with several thunderstorm north of the boundary (Fig. 7b). The convection continued to develop along a general west to east line near KTLX despite the fact that the larger outflow boundary had raced 1 1 hpa manual analysis of AWIPS surface data from 0100 to 0900 UTC implied this. But lacking sufficient pressures at all stations are not shown.

4 to the south (Fig. 7c). Over time the convection became weaker but persisted for hours as new cells trained over the region producing the similarly oriented band of heavy rainfall (Fig. 6). Without looping the data it is difficult to show how cells in a wave-like pattern trained from WNW to ESE over the region of heavy rainfall near the KTLX radar. 4. Forecasts The SREF is presented here despite relatively similar forecasts from other NCEP systems. The PW (Fig. 8) forecasts and 850 v-wind forecasts from 6 SREFS showed the surge of high PW air and the strong southerly to south-southwesterly jet over the region (Fig. 9). The PW field also showed the region of very dry air over western Oklahoma, an ingredient often associated with EMLs and significant severe weather events. The forecasts of PW valid at 0000 UTC 1 June (Fig. 10) showed an enhancement in the PW over eastern Oklahoma ahead of the implied dry line. The QPFs from the SREFs showing the probability of 100 mm or more QPF showed hints of the low probability event over portions of Missouri and Oklahoma in the 24 hour period of 1200 UTC 31May to 1 June (Fig. 11). Refining this to the approximate overnight period UTC 1 June showed at least a 50% chance of over 50 mm or rainfall in most model runs. With some decreased probabilities in shorter range forecasts (Fig. 12). The 25mm forecast also homed in on the rain in the southern plains (Fig. 13). All thresholds showed some threat for a moderate to heavy rainfall event, given no a priori knowledge of what a big rainfall event is in the NCEP SREF. Finally, the SREF mean QPF and 50 mm contours are shown in Figure 13. These data show the uncertainty with rainfall amounts in excess of 50 mm; the pattern in the overall rainfall; and more importantly, they show that many members had tight ovals of over 50 mm of rainfall indicative of convection. The variable location of each members convective evolutions precluded high probabilities of thresholds over 50 and 100mm but showed the convective initiation issue as a forecast issue related to the uncertainty. 5. Summary The convective event of 31 May 2013 will likely be remembered by many people for the deadly El Reno EF5 tornado, not the heavy rainfall that affected central Oklahoma as the supercell event transitioned into a heavy rainfall event (Fig. 1). After the severe threat diminished the threat of flooding increased in portions of eastern Oklahoma. The heavy rains in the region occurred in pattern which featured a strong low-level 850 hpa jet and a surge of above normal PW air into the region. Past work by Bodner et al. (2012) showed that the larger scale pattern was one which is observed in many heavy rainfall events. It was within this larger scale pattern that the heavy rains; mostly convectively driven; were observed.

5 The convective event over Oklahoma was clearly associated with a meso-high (see radar images Fig. 7 & Fig. 15) and would likely fall into the meso-high event category. As such, rapidly updated local scale models at 3-4km resolution would likely be the ideal way to examine events such as this. Despite the mesoscale nature of the event, the NCEP SREF and other models (not shown) indicated the potential for heavy rainfall in the region affected by the heavy rains. These systems generally lacked the details of the heavy rainfall pattern; though they provided valuable clues. 6. Acknowledgements Albany map for information and data on the storm. 7. References Banacos, Peter C., Michael L. Ekster, 2010: The Association of the Elevated Mixed Layer with Significant Severe Weather Events in the Northeastern United States. Wea. Forecasting, 25, LINK Bodner, M. J., N. W. Junker, R. H. Grumm, and R. S. Schumacher, 2011: Comparison of atmospheric circulation patterns during the 2008 and 1993 historic Midwest floods. Natl. Wea. Dig., 35, Carlson, T. N., and F. H. Ludlam, 1968: Conditions for the occurrence of severe local storms. Tellus, 20, Dry-line) Carlson, T. N., S. G. Benjamin, G. S. Forbes, and Y. F. Li, 1983: Elevated mixed layers in the regional severe storm environment: Conceptual model and case studies. Mon. Wea. Rev., 111, Fawbush, E. J., and R. C. Miller, 1954: The types of air masses in which North American tornadoes form. Bull. Amer. Meteor. Soc.35, (loaded gun/mixed layers) Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS. Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H. Bredekamp, NASA, Washington, D.C., Fawbush, E. J., and R. C. Miller, 1954: The types of air masses in which North American tornadoes form. Bull. Amer. Meteor. Soc.35, Grumm, R.H. 2011: New England Record Maker rain event of March NWA,Electronic Journal of Operational Meteorology,EJ4. Junker, N. W., R. S. Schneider and S. L. Fauver, 1999: Study of heavy rainfall events during the Great Midwest Flood of 1993.Wea. Forecasting, 14,

6 Junker, N.W, M. J. Brennan, F. Pereira, and M. Bodner, 2009: Assessing the potential for rare precipitation events with standardized anomalies and ensemble guidance at the Hydrometeorological Prediction Center. Bull. Amer. Meteor. Soc., 90, Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40- Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, Saha, Suranjana, et. al., 2010: The NCEP Climate Forecast System Reanalysis. Bull. Amer. Meteor. Soc., In Press (DOI: /2010BAMS3001.1). Schumacher, R. S. and R. H. Johnson, 2005: Organization and environmental properties of extreme-rain-producing mesoscale convective systems. Mon. Wea. Rev., 133, Stuart, N. and R. Grumm 2009, "The Use of Ensemble and Anomaly Data to Anticipate Extreme Flood Events in the Northeastern United States",NWA Digest,33,

7 Figure 1. Stage-IV estimated precipitation for the 36-hour period of 0000 UTC 31 May through 1200 UTC 1 June Values in milli-meters. Return to text..

8 Figure 2. CFS 500 hpa heights (m) and 500 hpa height anomalies (standard deviations) in 6-hour periods from a) 0000 UTC 31 May 2013 through f) 0600 UTC 01 June Return to text.

9 Figure 3. As in Figure 2 except for precipitable water (mm) and precipitable water anomalies. Return to text.

10 Figure 4. Storm Reports by type from the Storm Prediction Center (SPC) for 30 and 31 May Return to text.

11 Figure 5. As in Figure 3 except 850 hpa winds and 850 hpa v-wind anomalies. Return to text.

12 Figure 6. NMQ radar and gauge adjusted rainfall estimates for the 24 and 6-hour periods ending at 1000 UTC 1 June Return to text.

13 Figure 7a. KTLX radar at 0109 UTC showing 0.5 degree velocity and reflectivity. The yellow segment outlines the larger scale outflow from the convection. Return to text..

14 Figure 7b as in 7a except for 0118 UTC. Return to text. NWS State College Case Examples

15 Figure 7c. As in Figure 7a except valid at 0332 UTC. Return to text.. NWS State College Case Examples

16 Figure 8. SREF forecasts of PW and PW anomalies valid at 2100 UTC 31 May 2013 from forecasts initialized at a) 0300 UTC 29 May, b) 2100 UTC 29 May, c) 0900 UTC 30 May, d) 1500 UTC 30 May, e) 2100 UTC 30 May and f) 0300 UTC 31 May Return to text.

17 Figure 15. Meso analysis of surface features ever 1 hpa at 0300 UTC 01 June Courtesy of John LaCorte. Return to text.

18 Figure 9. As in Figure 8 except for 850 hpa winds. Return to text. NWS State College Case Examples

19 Figure 10. As in Figure 8 except valid time is 0000 UTC 01 June Return to text.

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21 Figure 11. As in Figure 8 except SREF probability of 100 mm (4 inches) or more QPF in the 24 hour period ending at 1200 UTC 01 June Return to text.

22 Figure 12. As in Figure 11 except for the probability of 50 mm or more QPF in the 12 hour period ending 1200 UTC 1 June Return to text.

23 Figure 13. As in Figure 12 except for 25 mm or QPF. Return to text. NWS State College Case Examples

24 Figure 14. As in Figure 12 except for mean QPF (shaded) from all members and each member 50mm contour. Return to text.